The present invention relates to a semiconductor device, and in particular to a semiconductor device utilizing quantum effect.
A semiconductor device having a quantum microstructure such as quantum well, quantum wire, quantum box, etc. is preferably used as a semiconductor light emitting device, and it exhibits excellent characteristics such as low threshold current, high modulation band, high coherence characteristics, etc. through quantum effect of electrons and holes. Further, such effects are remarkably noted in case distortion lower than critical film thickness is introduced to an active layer.
The methods to prepare quantum wires are as follows: a method to prepare quantum well structure and to obtain quantum wires by combining fine photolithography using electron beam exposure with vertical etching using ion beam, and a method to provide a V-shaped groove on a substrate as shown in FIG. 8 and to grow double hetero structure over the entire substrate surface having a groove with V-shaped cross-section (hereinafter referred as "V-groove").
However, the former method is disadvantageous in that side walls of the groove may be extensively damaged during fabrication and the quantum wires are often in poor quality.
On the other hand, the latter method has advantages in that quantum wires ran be selectively prepared by utilizing dependency of the growing speed on orientation, while it has drawbacks in that the bottom of the formed V-groove may be rounded depending upon the composition of the portion to form the V-groove, or oxide film is formed on the etching surface or contamination is caused by impurities during wet etching, or the bottom of V-groove is rounded due to etching.
Because quantum wire is a minute area, optical density on the light outgoing side tends to increase extremely when applied on laser structure, and only very weak light output can be obtained.
In a microstructure such as quantum wire, more quantum effects are present in thick film than in plane, and this is convenient to have high gain. When it is attempted to grow a distortion active layer with lattice mismatch ratio of 1 to 2% where the effect of distortion appears on flat substrate surface, the film thickness, at which dislocation starts to occur, i.e. critical film thickness, is only in the order of 10 to 20 nm because stress is applied on all over the surface of the distortion active layer. Thus, it is often not possible to have thick distortion active layer when it is to be utilized as quantum wire.
For this reason, it is an object of the present invention to provide a structure, in which quantum wires with good quality can be easily obtained, and a method to prepare such a structure.
It is another object of the present invention to improve efficiency of a semiconductor device.